1. Field of the Art
The present invention relates to a process for regenerating a Mo.Bi-based multi-oxide catalyst which has been used in a specific reaction, i.e., a reaction of formation of acrolein or methacrolein by vapor-phase catalytic oxidation of propylene or isobutene.
Mo.Bi-based multi-oxide catalysts are widely known to be useful for selective oxidation reaction of propylene or isobutene into acrolein or methacrolein, respectively, ammoxidation reaction of propylene and ammonia, or isobutene and ammonia into acrylonitrile or methacrylonitrile, respectively, and oxidative dehydrogenation reaction of butane into butadiene, and are being industrially used.
When, among these reactions, the selective oxidation reaction of olefins to aldehydes is compared with the ammoxidation reaction of olefins to nitriles, it may be said that the former is generally carried out at a temperature of approximately 260.degree. C. to 360.degree. C. in the presence of a catalyst in the form of a fixed bed, while the latter is generally carried out at a temperature of approximately 360.degree. C. to 460.degree. C. in the presence of a catalyst in the form of a fluidized bed. This difference in operational method is attributable to the characteristics of each reaction.
The catalyst to be subjected to such vapor phase catalytic reaction is used for a relatively long period of time. When the catalytic performance of the catalyst degenerates or deteriorates to a certain degree, the used catalyst is replaced with a fresh catalyst from an economical point of view. The used catalyst is ordinarily subjected to a process for recovering the metal contained therein.
It is well known that the deterioration of the catalytic performance of Mo Bi-based multi-oxide catalysts used for these vapor phase catalytic reactions has almost no causative relation to the deposition of coke on the catalysts but occurs mainly because of the loss of Mo by sublimation.
2. Prior Art
For these reasons, care has been taken in industrial processes using Mo.Bi-based multi-oxide catalysts to carry out measures such as reducing the reaction temperature as much as possible and reducing the partial pressure of steam as much as possible. Methods have also been proposed to regenerate the deteriorated Mo.Bi-based multi-oxide catalysts. That is, these methods are described in Japanese Patent Laid-Open Publication Nos. 50-49201, and 57-56044, Japanese Patent Publication No. 55-49541, U.S. Pat. No. 4,052,332, West Germany Patent Laid-Open Publication No. 3,311,521, and East Germany Pat. No. 137,889.
It is understood that these methods are all directed to catalysts used in an ammoxidation reaction, and it is considered logical that these methods aim at regenerating such catalysts. If it is supposed that the catalyst degeneration is due to the sublimation of Mo, the ammoxidation reaction must involve remarkable sublimation of Mo because it is carried out at a relatively high temperature as described hereinabove.
These regeneration methods are all characterized in that Mo in an amount corresponding to that of Mo lost by sublimation from the catalyst is added to the deteriorated catalyst by any suitable method, and the resultant catalyst is subjected to heat treatment, or Mo in an amount corresponding to that of Mo lost is added to the deteriorated catalyst together with other catalytic components (Bi, Cr, Mn, Fe, Co, Ni, etc.), and the resultant catalyst is subjected to heat treatment. Such a regeneration principle can be well understood in consideration of the fact that the main cause of the catalyst deterioration resides in sublimation of Mo.
However, as far as we know, it is considered that these regeneration methods are not always satisfactory from the point of view of industrial practice.